Skip, the important thing--as far as the manufacturer is concerned--is that they're all shorter than 2 3/4".
With some older Brit (and, I think, other European) doubles, you can have a problem with 2 3/4" hulls. It's been reported in at least a couple different places. Usually occurs on 19th century guns, which are more likely to have very short and sharply-taspered forcing cones. With those you can get blown ends and notice an increase in recoil. In those cases in particular, unless you want to have the cones lengthened, you're probably well advised to stick with 2 1/2" hulls.
